1. Field of the Invention
The present invention relates to a method of performing high efficiency coding of an image signal such as a video signal for a television and a system therefor, and more particularly, to a method and apparatus for performing optimal coding of the image signal depending upon the property thereof.
2. Description of the Related Art
Currently, a video signal for television, i.e., a TV signal is transmitted in an analog form. However, a digital transmission technique has being investigated and brought into practical use in a TV telephone system and so on. The digital transmission has features in that very few deterioration occurrences in quality is present in the midway of transmission and a long distance transmission is permitted. Also, a transmission path can be effectively utilized. For these reason, it is thought that the digitization of the transmission techniques will be advanced in future, and the transmission of the TV signal will also be digitized.
When the TV signal is concerned, however, an extremely large amount of information must be transmitted if it is digitized as it is and digitally transmitted, which requires a faster transmission apparatus, extension of the width of transmission paths and so on, thereby rendering the transmission cost too high and such digitization unrealistic.
When an image of a previous frame/field is compared with an image of a current frame/field, a different portion between them is often only very small fragments of them. In such a case, it is not efficient to repeatedly transmit image information of the same portion. If image information of the different portion alone is transmitted and an image of a current frame/field is produced from image information of a previous frame/field and the image information of the different portion, the amount of information to be transmitted can be reduced. The time-wise redundancy can thereby be eliminated in the transmission of the image signal. Also, assignment of a same code to each of adjacent pixels having substantially the same amplitude in the image signal for a frame/field portion results in wasting the information capacity. If the number of codes or the number of bits of codes assigned to these pixels can be reduced, the transmission efficiency can be improved. In other words, the spatial redundancy can be removed from an image signal. Thus, a conventional high efficiency coding system 100 for an image signal, which can remove the time-wise and spatial redundancies, is shown in FIG. 1.
Referring to FIG. 1, a subtractor 101 generates a prediction error signal by calculating the difference between a current image signal and a prediction image signal. A controller 102 is supplied with the current image signal for one of a plurality of blocks and the prediction image signal to compare their power amounts, and generates a mode signal in accordance with the comparison result. The mode signal indicates an intra mode or an inter mode. A switch 103 operates in response to the mode signal, i.e., selects, as a signal to be processed, the current image signal in the intra mode and the prediction error signal in the inter mode and outputs the selected signal to a circuit 105 for two-dimensional discrete cosine transform (2D-DCT). The circuit 105 executes 2D-DCT processing to the signal to be processed to generate transform coefficients. The transform coefficients are quantized by a circuit (Q) 106. The quantization coefficients are supplied to a scanning circuit 107 and a dequantization circuit (Q.sup.-1) 109. The quantization coefficients are scanned by the scanning circuit 107 in a zig-zag manner and read out as a train of coefficients to be supplied to a variable length coding circuit (VLC) 108. The VLC 108 codes the mode signal and each coefficient in the train and outputs them as a transmission signal. The quantization coefficients inversely quantized or dequantized by a circuit 109 go through inverse 2D-DCT processing executed by a circuit 110, whereby a signal to be inputted to the circuit 105 is substantially reproduced. The reproduced signal is supplied to an adder 111. A switch 104 operates in response to the mode signal such that it opens in the intra mode and closes in the inter mode to supply a signal from a predicting circuit 112 to the adder 111. As a result, the adder 111 outputs to the predicting circuit 112 the reproduced signal by the circuit 110 as a reproduced previous image signal for the next image signal in the intra mode, while reproduces a previous image signal for the next image signal from the reproduced signal as a prediction error signal and a previous image signal for the current image signal and outputs the previous image signal to the predicting circuit 112 in the inter mode. The predicting circuit 112 receives the next image signal, in addition to the reproduced previous image signal for the next image signal, generates a motion compensated signal from both signals, and outputs the motion compensated signal as the prediction image signal to the subtractor 101.
As described above, the conventional high efficiency coding system 100 removes the time-wise and spatial redundancies to some degree. However, when the output from the VLC 108 was examined, it was revealed that coding was not so efficiently carried out, or the redundancies were sometimes increased on the contrary.